1. Field of the Invention
The present invention relates to a printed circuit board and soldering structure assuring complete soldering of the terminal leads of electric or electronic parts or devices to its circuit pattern.
2. Related Art
Large-sized condensers are mounted onto a printed circuit board with their terminal leads soldered to selected conductors of the printed circuit. When a large-sized condenser whose diameter is 20 or more millimeters is soldered to a printed circuit, soldering trouble is often caused. Referring to FIG. 4, such a large-sized condenser 51 is put on a printed circuit board 52 with its terminal leads 53 in the terminal holes 54, and then, the terminal leads 53 are soldered to selected conductors 55 of the printed circuit pattern.
As seen from the drawing, the terminal leads 53 are bent in the terminal holes 54 to be caught and fixedly held in the holes 54, lest the condenser 51 should fall off when the printed circuit board 52 is inclined or turned upside down, or lest the condenser 52 should float when the printed circuit board 52 is dipped in the melted solder bath. As is well known, the printed circuit board 52 is brought to the flux bath where flux is applied to the bent terminal leads 53 of the condenser 51 for the purpose of preventing their oxidization, and then they are dried and heated preliminary to soldering. Then, the printed circuit board passes through the dip-soldering bath.
The flux is diluted to be low in viscosity, so that the flux may rise up in the terminal holes 54 under the influence of capillary attraction. Thus, the flux oozes out of the terminal holes 54 to form a flux layer 57 on the board surface in the space 58 delimited by the circular leg 56 of the condenser 51. As a result the substrate-and-condenser interspace 58 is sealed to be airtight.
The printed circuit board 52 is heated in soldering the terminal leads 53 to selected conductors 55 in the printed circuit pattern, and then, the air confined in the airtight space 58 is expanded, and at the same time, the flux layer 57 is vaporized. The expanded air and gaseous flux makes a vent hole through the soldering filet in the terminal hole 54, thus making the soldering defective. When the entrapped air shrinks in the closed space 58, the melted solder is drawn from the terminal hole 54 to spread in the closed space 58 and bridge a gap between the terminal leads 53, thus establishing a short circuit therebetween.
In an attempt to solve such a problem, JP 7-10971(U) entitled “Electronic Parts Mounting Structure” proposes that: 1) a vent hole is made in a selected site of the substrate where an electrolytic condenser is to be mounted; 2) a notched spacer is laid between the substrate and the electrolytic condenser to allow the expanded air and gaseous flux to escape through the notch or cut of the spacer; and 3) a projection is formed on the substrate to raise the electrolytic condenser, thus defining the open space around the condenser bottom for degassing.
Such a vent hole cannot be large enough to allow the expanding air and vaporized gaseous flux to escape completely. Use of an extra spacer or projection on the substrate increases work and cost in manufacturing printed circuits.
Similar trouble will be caused in soldering to the printed circuit board any other electric or electronic part or device whose bottom is so shaped as to form a closed space between its bottom and the board surface. Even in soldering a flat-bottomed electric or electronic part or device having its terminal leads extending from the flat bottom, such as a LED, the flux rises up in the terminal holes under the influence of capillary attraction to form a flux layer between the substrate surface and the flat bottom of the overlying electric or electronic part or device. When dipped and heated in the melted solder bath, the flux layer is vaporized, and the gaseous flux makes a vent hole in the solder fillet of the terminal hole so that it may escape outside.
Referring to FIG. 5, JP 2000-196215(A) entitled “Printed Circuit Board” proposes that a number of strips 59 are arranged on the substrate to be sandwiched between the substrate and the overlying electric or electronic part or device, so that the vent gaps may be made around the bottom of the electric or electronic part or device. The total thickness of the strip and creamy solder coating printed on the strip need to provide a vent space large enough to permit complete degassing. Sometimes such spacer-and-solder coating is not thick enough, and then, incomplete degassing is caused with the result that the heated air and gas is ejected through the solder-filled terminal holes, leading to the incomplete soldering.
In view of the above one object of the present invention is to provide a printed circuit board which permits complete degassing from the space between overlying electronic parts or devices and the board surface, thereby assuring reliable soldering of the electric or electronic parts or devices to the printed circuit.
Another object of the present invention is to provide a soldering structure for soldering electric or electronic parts or devices to printed circuits.